The present invention concerns a method and an apparatus for checking a security element which is furnished with a plurality of magnetic materials of different coercive field strength.
From DE 10 2011 106 263 A1 is known a checking method for checking such security elements. The security elements to be checked are furnished in different regions with a first magnetic material having a first coercive field strength and a second magnetic material having a second coercive field strength that is smaller than the first coercive field strength. In some regions the two magnetic materials are also provided jointly. Said regions form a magnetic code which can be checked.
For checking, it is provided to first subject the security element to a first, stronger magnetic field which is strong enough to magnetize both magnetic materials in a first direction, i.e. which has a magnetic field strength that is greater than the two above-mentioned coercive field strengths. Subsequently, the security element is subjected to a second, weaker magnetic field which has a magnetic field strength that is smaller than the first coercive field strength and is greater than the second coercive field strength and accordingly aligns the magnetization of the second magnetic material in a direction different from the magnetization direction of the first magnetic material. Then the magnetization of the security element is measured in a spatially resolved manner. From the thereby captured magnetic signal it is then derived where magnetic regions having only the first magnetic material and magnetic regions having only the second magnetic material are located. From the two different magnetic regions there are captured magnetic signals (e.g. magnetic flux densities or signals proportional thereto) which are substantially inverse to each other (i.e. signals with opposite signs).
When the magnetic materials of the two above-mentioned types mutually overlap, i.e. form a combined magnetic region having both magnetic materials, their magnetic signals can accordingly cancel each other out completely or nearly completely, so that such combined magnetic regions are not detected.
This problem is solved in DE 10 2011 106 263 by the security element being subsequently magnetized in a third magnetization direction as well, and further magnetic signals being captured on the basis of which the above-mentioned combined magnetic regions are also reliably detectable. This makes the checking method elaborate, however.